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NAME

       ssh-keygen — OpenSSH authentication key utility

SYNOPSIS

       ssh-keygen  [-q]  [-a  rounds] [-b bits] [-C comment] [-f output_keyfile] [-m format] [-N new_passphrase]
                  [-O option] [-t dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa] [-w provider] [-Z cipher]
       ssh-keygen -p [-a rounds] [-f keyfile] [-m format] [-N new_passphrase] [-P old_passphrase] [-Z cipher]
       ssh-keygen -i [-f input_keyfile] [-m key_format]
       ssh-keygen -e [-f input_keyfile] [-m key_format]
       ssh-keygen -y [-f input_keyfile]
       ssh-keygen -c [-a rounds] [-C comment] [-f keyfile] [-P passphrase]
       ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]
       ssh-keygen -B [-f input_keyfile]
       ssh-keygen -D pkcs11
       ssh-keygen -F hostname [-lv] [-f known_hosts_file]
       ssh-keygen -H [-f known_hosts_file]
       ssh-keygen -K [-a rounds] [-w provider]
       ssh-keygen -R hostname [-f known_hosts_file]
       ssh-keygen -r hostname [-g] [-f input_keyfile]
       ssh-keygen -M generate [-O option] output_file
       ssh-keygen -M screen [-f input_file] [-O option] output_file
       ssh-keygen -I certificate_identity -s ca_key [-hU]  [-D  pkcs11_provider]  [-n  principals]  [-O  option]
                  [-V validity_interval] [-z serial_number] file ...
       ssh-keygen -L [-f input_keyfile]
       ssh-keygen -A [-a rounds] [-f prefix_path]
       ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file ...
       ssh-keygen -Q [-l] -f krl_file file ...
       ssh-keygen -Y find-principals [-O option] -s signature_file -f allowed_signers_file
       ssh-keygen -Y match-principals -I signer_identity -f allowed_signers_file
       ssh-keygen -Y check-novalidate [-O option] -n namespace -s signature_file
       ssh-keygen -Y sign [-O option] -f key_file -n namespace file ...
       ssh-keygen   -Y   verify   [-O  option]  -f  allowed_signers_file  -I  signer_identity  -n  namespace  -s
                  signature_file [-r revocation_file]

DESCRIPTION

       ssh-keygen generates, manages and converts authentication keys for ssh(1).  ssh-keygen  can  create  keys
       for use by SSH protocol version 2.

       The  type  of  key  to  be  generated is specified with the -t option.  If invoked without any arguments,
       ssh-keygen will generate an RSA key.

       ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange  (DH-GEX).   See  the
       “MODULI GENERATION” section for details.

       Finally,  ssh-keygen  can  be used to generate and update Key Revocation Lists, and to test whether given
       keys have been revoked by one.  See the “KEY REVOCATION LISTS” section for details.

       Normally each user wishing to use SSH with public  key  authentication  runs  this  once  to  create  the
       authentication    key   in   ~/.ssh/id_dsa,   ~/.ssh/id_ecdsa,   ~/.ssh/id_ecdsa_sk,   ~/.ssh/id_ed25519,
       ~/.ssh/id_ed25519_sk or ~/.ssh/id_rsa.  Additionally, the system administrator may use this  to  generate
       host keys.

       Normally  this  program  generates  the  key  and asks for a file in which to store the private key.  The
       public key is stored in a file with the same name but “.pub” appended.   The  program  also  asks  for  a
       passphrase.   The  passphrase  may  be  empty  to  indicate  no  passphrase (host keys must have an empty
       passphrase), or it may be a string of arbitrary length.  A passphrase is similar to a password, except it
       can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of characters you
       want.  Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable
       (English prose has only 1-2 bits of entropy per  character,  and  provides  very  bad  passphrases),  and
       contain  a  mix of upper and lowercase letters, numbers, and non-alphanumeric characters.  The passphrase
       can be changed later by using the -p option.

       There is no way to recover a lost passphrase.  If the passphrase is lost or forgotten, a new key must  be
       generated and the corresponding public key copied to other machines.

       ssh-keygen  will  by  default  write  keys in an OpenSSH-specific format.  This format is preferred as it
       offers better protection for keys at rest as well as allowing storage of key comments within the  private
       key  file itself.  The key comment may be useful to help identify the key.  The comment is initialized to
       “user@host” when the key is created, but can be changed using the -c option.

       It is still possible for ssh-keygen to write the previously-used PEM format private  keys  using  the  -m
       flag.   This  may  be  used when generating new keys, and existing new-format keys may be converted using
       this option in conjunction with the -p (change passphrase) flag.

       After a key is generated, ssh-keygen will ask where the keys should be placed to be activated.

       The options are as follows:

       -A      For each of the key types (rsa, dsa, ecdsa and  ed25519)  for  which  host  keys  do  not  exist,
               generate  the host keys with the default key file path, an empty passphrase, default bits for the
               key type, and default comment.  If -f has also been specified, its argument is used as  a  prefix
               to  the  default  path  for  the resulting host key files.  This is used by system administration
               scripts to generate new host keys.

       -a rounds
               When saving a private key, this option specifies the number  of  KDF  (key  derivation  function,
               currently  bcrypt_pbkdf(3)) rounds used.  Higher numbers result in slower passphrase verification
               and increased resistance to brute-force password cracking  (should  the  keys  be  stolen).   The
               default is 16 rounds.

       -B      Show the bubblebabble digest of specified private or public key file.

       -b bits
               Specifies  the  number of bits in the key to create.  For RSA keys, the minimum size is 1024 bits
               and the default is 3072 bits.  Generally, 3072 bits is considered sufficient.  DSA keys  must  be
               exactly  1024  bits  as  specified by FIPS 186-2.  For ECDSA keys, the -b flag determines the key
               length by selecting from one of three elliptic curve sizes: 256, 384 or 521 bits.  Attempting  to
               use  bit  lengths  other than these three values for ECDSA keys will fail.  ECDSA-SK, Ed25519 and
               Ed25519-SK keys have a fixed length and the -b flag will be ignored.

       -C comment
               Provides a new comment.

       -c      Requests changing the comment in the private and public key files.  The program will  prompt  for
               the  file  containing  the  private  keys, for the passphrase if the key has one, and for the new
               comment.

       -D pkcs11
               Download the public keys provided by the PKCS#11 shared library pkcs11.  When used in combination
               with -s, this option indicates that a CA key resides in a PKCS#11 token (see  the  “CERTIFICATES”
               section for details).

       -E fingerprint_hash
               Specifies the hash algorithm used when displaying key fingerprints.  Valid options are: “md5” and
               “sha256”.  The default is “sha256”.

       -e      This  option  will  read a private or public OpenSSH key file and print to stdout a public key in
               one of the formats specified by the -m option.  The default export  format  is  “RFC4716”.   This
               option  allows exporting OpenSSH keys for use by other programs, including several commercial SSH
               implementations.

       -F hostname | [hostname]:port
               Search for the specified hostname (with optional port number) in a known_hosts file, listing  any
               occurrences  found.  This option is useful to find hashed host names or addresses and may also be
               used in conjunction with the -H option to print found keys in a hashed format.

       -f filename
               Specifies the filename of the key file.

       -g      Use generic DNS format when printing fingerprint resource records using the -r command.

       -H      Hash a known_hosts file.  This replaces all hostnames and addresses with  hashed  representations
               within  the  specified  file;  the original content is moved to a file with a .old suffix.  These
               hashes may be used normally by ssh and sshd, but  they  do  not  reveal  identifying  information
               should  the  file's contents be disclosed.  This option will not modify existing hashed hostnames
               and is therefore safe to use on files that mix hashed and non-hashed names.

       -h      When signing a  key,  create  a  host  certificate  instead  of  a  user  certificate.   See  the
               “CERTIFICATES” section for details.

       -I certificate_identity
               Specify the key identity when signing a public key.  See the “CERTIFICATES” section for details.

       -i      This  option will read an unencrypted private (or public) key file in the format specified by the
               -m option and print an OpenSSH compatible private (or public) key to stdout.  This option  allows
               importing  keys  from  other  software,  including  several  commercial SSH implementations.  The
               default import format is “RFC4716”.

       -K      Download resident keys from a FIDO authenticator.  Public and private key files will  be  written
               to  the current directory for each downloaded key.  If multiple FIDO authenticators are attached,
               keys will be downloaded from the first touched authenticator.

       -k      Generate a KRL file.  In this mode, ssh-keygen will generate a KRL file at the location specified
               via the  -f  flag  that  revokes  every  key  or  certificate  presented  on  the  command  line.
               Keys/certificates to be revoked may be specified by public key file or using the format described
               in the “KEY REVOCATION LISTS” section.

       -L      Prints the contents of one or more certificates.

       -l      Show fingerprint of specified public key file.  For RSA and DSA keys ssh-keygen tries to find the
               matching  public  key  file  and prints its fingerprint.  If combined with -v, a visual ASCII art
               representation of the key is supplied with the fingerprint.

       -M generate
               Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters  for  eventual  use  by  the
               ‘diffie-hellman-group-exchange-*’  key exchange methods.  The numbers generated by this operation
               must be further screened before use.  See the “MODULI GENERATION” section for more information.

       -M screen
               Screen candidate parameters for Diffie-Hellman Group  Exchange.   This  will  accept  a  list  of
               candidate  numbers  and  test  that  they  are safe (Sophie Germain) primes with acceptable group
               generators.  The results of this operation may be added to the  /etc/ssh/moduli  file.   See  the
               “MODULI GENERATION” section for more information.

       -m key_format
               Specify a key format for key generation, the -i (import), -e (export) conversion options, and the
               -p  change  passphrase  operation.  The latter may be used to convert between OpenSSH private key
               and PEM private key formats.  The supported key formats are: “RFC4716” (RFC 4716/SSH2  public  or
               private  key),  “PKCS8”  (PKCS8  public  or  private  key) or “PEM” (PEM public key).  By default
               OpenSSH will write newly-generated private keys in its own format,  but  when  converting  public
               keys  for  export  the default format is “RFC4716”.  Setting a format of “PEM” when generating or
               updating a supported private key type will cause the key to be stored in the legacy  PEM  private
               key format.

       -N new_passphrase
               Provides the new passphrase.

       -n principals
               Specify  one or more principals (user or host names) to be included in a certificate when signing
               a key.  Multiple principals may be  specified,  separated  by  commas.   See  the  “CERTIFICATES”
               section for details.

       -O option
               Specify  a  key/value  option.   These  are  specific  to  the operation that ssh-keygen has been
               requested to perform.

               When signing certificates, one of the  options  listed  in  the  “CERTIFICATES”  section  may  be
               specified here.

               When  performing  moduli  generation  or  screening,  one  of  the  options listed in the “MODULI
               GENERATION” section may be specified.

               When generating a key that will be hosted on a FIDO authenticator,  this  flag  may  be  used  to
               specify key-specific options.  Those supported at present are:

               application
                       Override  the  default FIDO application/origin string of “ssh:”.  This may be useful when
                       generating host or domain-specific resident keys.  The specified application string  must
                       begin with “ssh:”.

               challenge=path
                       Specifies  a  path to a challenge string that will be passed to the FIDO token during key
                       generation.  The challenge string may be used as part of an out-of-band protocol for  key
                       enrollment (a random challenge is used by default).

               device  Explicitly  specify  a  fido(4)  device  to use, rather than letting the token middleware
                       select one.

               no-touch-required
                       Indicate that the generated private key should not require touch events  (user  presence)
                       when making signatures.  Note that sshd(8) will refuse such signatures by default, unless
                       overridden via an authorized_keys option.

               resident
                       Indicate  that  the key should be stored on the FIDO authenticator itself.  Resident keys
                       may be supported on FIDO2 tokens and typically require that a PIN be  set  on  the  token
                       prior to generation.  Resident keys may be loaded off the token using ssh-add(1).

               user    A  username  to be associated with a resident key, overriding the empty default username.
                       Specifying a username may be useful when generating multiple resident keys for  the  same
                       application name.

               verify-required
                       Indicate  that this private key should require user verification for each signature.  Not
                       all FIDO tokens support this option.  Currently PIN authentication is the only  supported
                       verification method, but other methods may be supported in the future.

               write-attestation=path
                       May  be  used  at  key  generation time to record the attestation data returned from FIDO
                       tokens during key generation.  This information is potentially  sensitive.   By  default,
                       this information is discarded.

               When performing signature-related options using the -Y flag, the following options are accepted:

               hashalg=algorithm
                       Selects the hash algorithm to use for hashing the message to be signed.  Valid algorithms
                       are “sha256” and “sha512.” The default is “sha512.”

               print-pubkey
                       Print the full public key to standard output after signature verification.

               verify-time=timestamp
                       Specifies a time to use when validating signatures instead of the current time.  The time
                       may be specified as a date in YYYYMMDD format or a time in YYYYMMDDHHMM[SS] format.

               The -O option may be specified multiple times.

       -P passphrase
               Provides the (old) passphrase.

       -p      Requests  changing  the  passphrase  of a private key file instead of creating a new private key.
               The program will prompt for the file containing the private key,  for  the  old  passphrase,  and
               twice for the new passphrase.

       -Q      Test  whether  keys  have  been  revoked  in  a KRL.  If the -l option is also specified then the
               contents of the KRL will be printed.

       -q      Silence ssh-keygen.

       -R hostname | [hostname]:port
               Removes all keys belonging  to  the  specified  hostname  (with  optional  port  number)  from  a
               known_hosts file.  This option is useful to delete hashed hosts (see the -H option above).

       -r hostname
               Print the SSHFP fingerprint resource record named hostname for the specified public key file.

       -s ca_key
               Certify  (sign)  a  public  key  using  the specified CA key.  See the “CERTIFICATES” section for
               details.

               When generating a KRL, -s specifies a path to a CA public key file used  to  revoke  certificates
               directly by key ID or serial number.  See the “KEY REVOCATION LISTS” section for details.

       -t dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
               Specifies  the  type  of  key  to  create.   The  possible values are “dsa”, “ecdsa”, “ecdsa-sk”,
               “ed25519”, “ed25519-sk”, or “rsa”.

               This flag may also be used to specify the desired signature type when signing certificates  using
               an  RSA  CA  key.   The  available  RSA  signature  variants  are “ssh-rsa” (SHA1 signatures, not
               recommended), “rsa-sha2-256”, and “rsa-sha2-512” (the default).

       -U      When used in combination with -s, this option indicates that a CA key resides in a  ssh-agent(1).
               See the “CERTIFICATES” section for more information.

       -u      Update a KRL.  When specified with -k, keys listed via the command line are added to the existing
               KRL rather than a new KRL being created.

       -V validity_interval
               Specify  a  validity  interval  when signing a certificate.  A validity interval may consist of a
               single time, indicating that the certificate is valid beginning now and expiring at that time, or
               may consist of two times separated by a colon to indicate an explicit time interval.

               The start time may be specified as the  string  “always”  to  indicate  the  certificate  has  no
               specified  start  time,  a date in YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative
               time (to the current time) consisting of a minus sign followed  by  an  interval  in  the  format
               described in the TIME FORMATS section of sshd_config(5).

               The  end  time  may  be  specified  as  a YYYYMMDD date, a YYYYMMDDHHMM[SS] time, a relative time
               starting with a plus character or the string “forever” to indicate that the  certificate  has  no
               expiry date.

               For  example:  “+52w1d”  (valid from now to 52 weeks and one day from now), “-4w:+4w” (valid from
               four weeks ago to four weeks from now), “20100101123000:20110101123000”  (valid  from  12:30  PM,
               January  1st,  2010  to  12:30  PM,  January  1st, 2011), “-1d:20110101” (valid from yesterday to
               midnight, January 1st, 2011), “-1m:forever” (valid from one minute ago and never expiring).

       -v      Verbose mode.  Causes ssh-keygen to print debugging messages about its progress.  This is helpful
               for debugging moduli generation.  Multiple -v options increase the verbosity.  The maximum is 3.

       -w provider
               Specifies a path to a library that will be used when  creating  FIDO  authenticator-hosted  keys,
               overriding the default of using the internal USB HID support.

       -Y find-principals
               Find  the  principal(s) associated with the public key of a signature, provided using the -s flag
               in an authorized signers file provided using the -f flag.  The format of the allowed signers file
               is documented in the “ALLOWED SIGNERS” section below.  If one or  more  matching  principals  are
               found, they are returned on standard output.

       -Y match-principals
               Find  principal  matching the principal name provided using the -I flag in the authorized signers
               file specified using the -f flag.  If one  or  more  matching  principals  are  found,  they  are
               returned on standard output.

       -Y check-novalidate
               Checks  that a signature generated using ssh-keygen -Y sign has a valid structure.  This does not
               validate if a signature comes from an authorized signer.  When testing  a  signature,  ssh-keygen
               accepts  a  message  on standard input and a signature namespace using -n.  A file containing the
               corresponding signature must also be supplied using the  -s  flag.   Successful  testing  of  the
               signature is signalled by ssh-keygen returning a zero exit status.

       -Y sign
               Cryptographically  sign  a  file  or some data using a SSH key.  When signing, ssh-keygen accepts
               zero or more files to sign on the command-line - if no files are specified then  ssh-keygen  will
               sign data presented on standard input.  Signatures are written to the path of the input file with
               “.sig” appended, or to standard output if the message to be signed was read from standard input.

               The  key used for signing is specified using the -f option and may refer to either a private key,
               or a public key with the private  half  available  via  ssh-agent(1).   An  additional  signature
               namespace, used to prevent signature confusion across different domains of use (e.g. file signing
               vs  email  signing)  must be provided via the -n flag.  Namespaces are arbitrary strings, and may
               include: “file” for file signing, “email” for email signing.  For custom uses, it is  recommended
               to use names following a NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.

       -Y verify
               Request  to  verify  a  signature  generated  using  ssh-keygen -Y sign as described above.  When
               verifying a signature, ssh-keygen accepts a message on standard input and a  signature  namespace
               using -n.  A file containing the corresponding signature must also be supplied using the -s flag,
               along  with  the  identity  of the signer using -I and a list of allowed signers via the -f flag.
               The format of the allowed signers file is documented in the “ALLOWED SIGNERS” section  below.   A
               file  containing  revoked keys can be passed using the -r flag.  The revocation file may be a KRL
               or a one-per-line list of public keys.   Successful  verification  by  an  authorized  signer  is
               signalled by ssh-keygen returning a zero exit status.

       -y      This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.

       -Z cipher
               Specifies  the cipher to use for encryption when writing an OpenSSH-format private key file.  The
               list of available ciphers may be obtained using "ssh -Q cipher".  The default is “aes256-ctr”.

       -z serial_number
               Specifies a serial number to be embedded in the certificate to distinguish this certificate  from
               others  from the same CA.  If the serial_number is prefixed with a ‘+’ character, then the serial
               number will be incremented for each certificate signed on a  single  command-line.   The  default
               serial number is zero.

               When generating a KRL, the -z flag is used to specify a KRL version number.

MODULI GENERATION

       ssh-keygen  may  be  used  to  generate  groups  for the Diffie-Hellman Group Exchange (DH-GEX) protocol.
       Generating these groups is a two-step process: first, candidate primes are generated using  a  fast,  but
       memory  intensive  process.   These  candidate  primes  are  then tested for suitability (a CPU-intensive
       process).

       Generation of primes is performed using the -M generate option.  The desired length of the primes may  be
       specified by the -O bits option.  For example:

             # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates

       By  default,  the  search  for  primes begins at a random point in the desired length range.  This may be
       overridden using the -O start option, which specifies a different start point (in hex).

       Once a set of candidates have been generated, they  must  be  screened  for  suitability.   This  may  be
       performed  using  the -M screen option.  In this mode ssh-keygen will read candidates from standard input
       (or a file specified using the -f option).  For example:

             # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048

       By default, each candidate will be subjected to 100 primality tests.  This may be overridden using the -O
       prime-tests  option.   The  DH  generator  value  will  be  chosen  automatically  for  the  prime  under
       consideration.   If  a  specific generator is desired, it may be requested using the -O generator option.
       Valid generator values are 2, 3, and 5.

       Screened DH groups may be installed in /etc/ssh/moduli.  It is important that this file  contains  moduli
       of a range of bit lengths.

       A number of options are available for moduli generation and screening via the -O flag:

       lines=number
               Exit after screening the specified number of lines while performing DH candidate screening.

       start-line=line-number
               Start screening at the specified line number while performing DH candidate screening.

       checkpoint=filename
               Write  the  last  line  processed  to the specified file while performing DH candidate screening.
               This will be used to skip lines in the input file that have already been processed if the job  is
               restarted.

       memory=mbytes
               Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-GEX.

       start=hex-value
               Specify start point (in hex) when generating candidate moduli for DH-GEX.

       generator=value
               Specify desired generator (in decimal) when testing candidate moduli for DH-GEX.

CERTIFICATES

       ssh-keygen  supports  signing  of  keys  to  produce  certificates  that  may  be  used  for user or host
       authentication.  Certificates consist of a public key, some identity information, zero or more  principal
       (user or host) names and a set of options that are signed by a Certification Authority (CA) key.  Clients
       or  servers may then trust only the CA key and verify its signature on a certificate rather than trusting
       many user/host keys.  Note that OpenSSH certificates are a different, and much  simpler,  format  to  the
       X.509 certificates used in ssl(8).

       ssh-keygen  supports  two  types of certificates: user and host.  User certificates authenticate users to
       servers, whereas host certificates authenticate server hosts to users.  To generate a user certificate:

             $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub

       The resultant certificate will be placed in /path/to/user_key-cert.pub.  A host certificate requires  the
       -h option:

             $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub

       The host certificate will be output to /path/to/host_key-cert.pub.

       It  is  possible to sign using a CA key stored in a PKCS#11 token by providing the token library using -D
       and identifying the CA key by providing its public half as an argument to -s:

             $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub

       Similarly, it is possible for the CA key to be hosted in a ssh-agent(1).  This is  indicated  by  the  -U
       flag and, again, the CA key must be identified by its public half.

             $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub

       In  all cases, key_id is a "key identifier" that is logged by the server when the certificate is used for
       authentication.

       Certificates may be limited to be valid for a set of principal (user/host) names.  By default,  generated
       certificates  are  valid  for  all  users  or  hosts.   To  generate a certificate for a specified set of
       principals:

             $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
             $ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub

       Additional limitations on the validity and use of user certificates may be specified through  certificate
       options.   A certificate option may disable features of the SSH session, may be valid only when presented
       from particular source addresses or may force the use of a specific command.

       The options that are valid for user certificates are:

       clear   Clear all enabled permissions.  This is useful for clearing the default  set  of  permissions  so
               permissions may be added individually.

       critical:name[=contents]
       extension:name[=contents]
               Includes  an  arbitrary  certificate  critical  option  or  extension.  The specified name should
               include a domain suffix, e.g. “name@example.com”.  If contents is specified then it  is  included
               as  the  contents  of the extension/option encoded as a string, otherwise the extension/option is
               created with no contents (usually indicating a flag).  Extensions may be ignored by a  client  or
               server  that does not recognise them, whereas unknown critical options will cause the certificate
               to be refused.

       force-command=command
               Forces the execution of command instead of any shell or command specified by the  user  when  the
               certificate is used for authentication.

       no-agent-forwarding
               Disable ssh-agent(1) forwarding (permitted by default).

       no-port-forwarding
               Disable port forwarding (permitted by default).

       no-pty  Disable PTY allocation (permitted by default).

       no-user-rc
               Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

       no-x11-forwarding
               Disable X11 forwarding (permitted by default).

       permit-agent-forwarding
               Allows ssh-agent(1) forwarding.

       permit-port-forwarding
               Allows port forwarding.

       permit-pty
               Allows PTY allocation.

       permit-user-rc
               Allows execution of ~/.ssh/rc by sshd(8).

       permit-X11-forwarding
               Allows X11 forwarding.

       no-touch-required
               Do  not  require  signatures  made using this key include demonstration of user presence (e.g. by
               having  the  user  touch  the  authenticator).   This  option  only  makes  sense  for  the  FIDO
               authenticator algorithms ecdsa-sk and ed25519-sk.

       source-address=address_list
               Restrict  the  source addresses from which the certificate is considered valid.  The address_list
               is a comma-separated list of one or more address/netmask pairs in CIDR format.

       verify-required
               Require signatures made using this key indicate that the user was first  verified.   This  option
               only  makes  sense  for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.  Currently PIN
               authentication is the only supported verification method, but other methods may be  supported  in
               the future.

       At present, no standard options are valid for host keys.

       Finally,  certificates  may  be  defined with a validity lifetime.  The -V option allows specification of
       certificate start and end times.  A certificate that is presented at a time outside this range  will  not
       be considered valid.  By default, certificates are valid from the Unix Epoch to the distant future.

       For certificates to be used for user or host authentication, the CA public key must be trusted by sshd(8)
       or ssh(1).  Refer to those manual pages for details.

KEY REVOCATION LISTS

       ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).  These binary files specify keys
       or certificates to be revoked using a compact format, taking as little as one bit per certificate if they
       are being revoked by serial number.

       KRLs  may  be generated using the -k flag.  This option reads one or more files from the command line and
       generates a new KRL.  The files may either contain a KRL specification (see below) or public keys, listed
       one per line.  Plain public keys  are  revoked  by  listing  their  hash  or  contents  in  the  KRL  and
       certificates revoked by serial number or key ID (if the serial is zero or not available).

       Revoking  keys  using a KRL specification offers explicit control over the types of record used to revoke
       keys and may be used to directly revoke certificates by serial  number  or  key  ID  without  having  the
       complete  original  certificate  on  hand.   A  KRL specification consists of lines containing one of the
       following directives followed by a colon and some directive-specific information.

       serial: serial_number[-serial_number]
               Revokes a certificate with the specified serial number.  Serial numbers are  64-bit  values,  not
               including  zero  and  may  be  expressed  in  decimal,  hex  or octal.  If two serial numbers are
               specified separated by a hyphen, then the range of serial numbers including and between  each  is
               revoked.  The CA key must have been specified on the ssh-keygen command line using the -s option.

       id: key_id
               Revokes  a  certificate with the specified key ID string.  The CA key must have been specified on
               the ssh-keygen command line using the -s option.

       key: public_key
               Revokes the specified key.  If a certificate is listed, then it is revoked as a plain public key.

       sha1: public_key
               Revokes the specified key by including its SHA1 hash in the KRL.

       sha256: public_key
               Revokes the specified key by including its SHA256 hash in the KRL.   KRLs  that  revoke  keys  by
               SHA256 hash are not supported by OpenSSH versions prior to 7.9.

       hash: fingerprint
               Revokes  a key using a fingerprint hash, as obtained from a sshd(8) authentication log message or
               the ssh-keygen -l flag.  Only SHA256 fingerprints are supported here and resultant KRLs  are  not
               supported by OpenSSH versions prior to 7.9.

       KRLs  may be updated using the -u flag in addition to -k.  When this option is specified, keys listed via
       the command line are merged into the KRL, adding to those already there.

       It is also possible, given a KRL, to test whether it revokes a particular key (or  keys).   The  -Q  flag
       will  query  an  existing  KRL, testing each key specified on the command line.  If any key listed on the
       command line has been revoked (or an error encountered) then ssh-keygen will exit with  a  non-zero  exit
       status.  A zero exit status will only be returned if no key was revoked.

ALLOWED SIGNERS

       When  verifying  signatures,  ssh-keygen uses a simple list of identities and keys to determine whether a
       signature comes from an authorized source.  This "allowed signers" file uses a format patterned after the
       AUTHORIZED_KEYS FILE FORMAT described in sshd(8).  Each line of the file contains  the  following  space-
       separated  fields: principals, options, keytype, base64-encoded key.  Empty lines and lines starting with
       a ‘#’ are ignored as comments.

       The principals field is a pattern-list (see PATTERNS in ssh_config(5)) consisting of one or  more  comma-
       separated  USER@DOMAIN  identity  patterns  that  are accepted for signing.  When verifying, the identity
       presented via the -I option must match a principals pattern in order for  the  corresponding  key  to  be
       considered acceptable for verification.

       The  options  (if  present)  consist  of comma-separated option specifications.  No spaces are permitted,
       except within double quotes.  The  following  option  specifications  are  supported  (note  that  option
       keywords are case-insensitive):

       cert-authority
               Indicates  that this key is accepted as a certificate authority (CA) and that certificates signed
               by this CA may be accepted for verification.

       namespaces=namespace-list
               Specifies a pattern-list of namespaces that are  accepted  for  this  key.   If  this  option  is
               present,  the  signature  namespace  embedded  in  the  signature  object  and  presented  on the
               verification command-line must match the  specified  list  before  the  key  will  be  considered
               acceptable.

       valid-after=timestamp
               Indicates  that the key is valid for use at or after the specified timestamp, which may be a date
               in YYYYMMDD format or a time in YYYYMMDDHHMM[SS] format.

       valid-before=timestamp
               Indicates that the key is valid for use at or before the specified timestamp.

       When verifying signatures made  by  certificates,  the  expected  principal  name  must  match  both  the
       principals pattern in the allowed signers file and the principals embedded in the certificate itself.

       An example allowed signers file:

          # Comments allowed at start of line
          user1@example.com,user2@example.com ssh-rsa AAAAX1...
          # A certificate authority, trusted for all principals in a domain.
          *@example.com cert-authority ssh-ed25519 AAAB4...
          # A key that is accepted only for file signing.
          user2@example.com namespaces="file" ssh-ed25519 AAA41...

ENVIRONMENT

       SSH_SK_PROVIDER
               Specifies  a path to a library that will be used when loading any FIDO authenticator-hosted keys,
               overriding the default of using the built-in USB HID support.

FILES

       ~/.ssh/id_dsa
       ~/.ssh/id_ecdsa
       ~/.ssh/id_ecdsa_sk
       ~/.ssh/id_ed25519
       ~/.ssh/id_ed25519_sk
       ~/.ssh/id_rsa
               Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
               authentication identity of the user.  This file should not be readable by anyone  but  the  user.
               It  is  possible to specify a passphrase when generating the key; that passphrase will be used to
               encrypt the private part of this file using 128-bit AES.  This file is not automatically accessed
               by ssh-keygen but it is offered as the default file for the private key.  ssh(1) will  read  this
               file when a login attempt is made.

       ~/.ssh/id_dsa.pub
       ~/.ssh/id_ecdsa.pub
       ~/.ssh/id_ecdsa_sk.pub
       ~/.ssh/id_ed25519.pub
       ~/.ssh/id_ed25519_sk.pub
       ~/.ssh/id_rsa.pub
               Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
               public   key   for   authentication.    The   contents   of   this   file   should  be  added  to
               ~/.ssh/authorized_keys on all machines  where  the  user  wishes  to  log  in  using  public  key
               authentication.  There is no need to keep the contents of this file secret.

       /etc/ssh/moduli
               Contains Diffie-Hellman groups used for DH-GEX.  The file format is described in moduli(5).

SEE ALSO

       ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

       The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

AUTHORS

       OpenSSH  is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Campbell, Bob
       Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer  features
       and created OpenSSH.  Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

Debian                                          February 6, 2022                                   SSH-KEYGEN(1)